I've come across mention of a Certificate IV in TESOL in these forums and on some ESL schools' websites. I just read a course description of the Cert. IV and it sounds like it covers much the same area as that in a CELTA.

What's the difference? Is one a continuation of the other? Is one certificate preferred over the other? In Australia? Around the world?

Only the Cambridge Celta and Trinity Tesol have so far achieved world-wide recognition. But if you think that the Certificate IV is a good course you could highlight it's contents when applying to prospective employers.

The CELTA may help you get a language school job in Australia, but ELICOS and state/private school work would need a PGCTESOL at minimum, and of course a teaching qualification. A PGTESOL is one semester full time, delivered by a tertiary institute. A CELTA is one month, delivered by private providers. The maths should tell you which has the higher status. This is not to say the CELTA is inferior in content or delivery though, as many experienced teachers speak highly of it.
The PGC qualifies Australian residents for study loans and income support: I'm not sure about CELTA.
I did the PGC while overseas, and didn't have to repay until I was back home in full time work.

I've got a different kind of Certificate IV which applies to teaching adults, and anywhere outside Australia that I mention it to employers, my remarks are met with a "You've got a what??". Its completely useless outside Australia, even if I give a full description of everything that is covered on the course.

So there you go: it depends where you want to work.
I got a job application package for an ELICOS position at an Adelaide uni. today. Minimum qual. was a post-grad TESOL, preferably at masters level. The salary is $48k; not great, but not bad.
CELTA is good for working the EFL market. If you want to work in an English speaking country, take the PG pathway.